This invention relates to electrically operated fastener driving tools and, more particularly, to devices of this type which are provided with electronic control circuitry for supplying a predetermined plurality of unidirectional electronic impulses to a solenoid which powers the fastener driving blade of the device. This results in the delivery of a like number of driving strokes to a single fastener for each actuation of the tool. Means is also provided for preventing the advancement of more than one fastener into the path of the driver blade during the driving strokes produced in a single actuation of the device.
Electronically operated fastener driving tools are disclosed in U.S. Pat. No. 3,971,969 and in patents cited therein. The cited patent relates to electrically operated stapling devices and, in particular, to devices which are provided with electronic control circuitry for supplying unidirectional electronic impulses for operating the staple driving blade of the device. In the cited patent the driving power is delivered to the blade in two or more pulses corresponding to at least two alternate half-cycles of opposite polarity from an alternating power source.
An electronic trigger circuit is disclosed in the General Electric SCR Manual, Fifth Edition, at pp. 202-3 (1972). The GE SCR Manual discloses a one-shot trigger control circuit which triggers an SCR for one complete half-cycle only of an AC supply. The SCR Manual teaches that the disclosed circuit may be utilized for the solenoid drives of electrically operated tools where load current must flow for one complete half-cycle only to produce a single stroke of a solenoid armature regardless of the length of time the trigger of the device is maintained in the "on" position.
It has been found to be advantageous to operate an electronic fastener driving tool in such a manner that each fastener is driven with more than one stroke of the driver blade. The fastener is thus completely set in the workpiece and less energy is expended per stroke of the driver blade rendering the entire operation quieter and resulting in less wear and tear on the device to thereby extend its useful life. By providing multiple driving strokes to each fastener, penetration of harder working surfaces than that normally possible with single stroke devices is accomplished.
The present invention utilizes circuitry comprising only diodes, resistors, capacitors and a single SCR to provide a predetermined plurality of unidirectional current pulses to the solenoid during consecutive like-poled half-cycles of alternating current so that the driver blade will deliver a predetermined plurality of driving strokes, preferably two, for a single actuation of the tool. Alternative mechanical means are also provided, responsive to the activating mechanism of the tool, to prevent more than one fastener, in a strip of fasteners, for example, from being advanced into the path of the driver blade during a single actuation of the tool. In one embodiment, a damper assembly, also referred to herein as a dashpot assembly, is disclosed in which the return of the driver blade to its normal rest position is restrained and the driver blade itself prevents the travel of the following fasteners into the drive track. In an alternative embodiment, a clamping assembly responsive to the actuation of trigger means physically clamps down on the first following fastener and prevents its movement into the drive track. By either of these alternative mechanical means there is prevented the possibility of more than one fastener from being driven as a result of the multiple stroke action of the driver blade.